The present invention relates to a lens or objective for a camera, more particularly for a digital camera, comprising a housing, an actuating element arranged on the housing, and a lens element system that can be set into a plurality of settings, wherein the lens element system is embodied in such a way that in at least one setting an f-number is F≦3.
In cameras that can be set to a small f-number, one of the significantly occurring aberrations is the so-called aperture aberration. The aperture aberration is the so-called spherical aberration. The aperture aberration is all the greater, the further a light beam proceeding from an object point is fanned out in an optical system of a lens before it impinges on the image plane. The rays incident through ring zones that are spaced apart from the optical axis at a distance h intersect at a different image point on the optical axis from the paraxial image point. The aperture aberration can be expressed by the so-called intersection length difference, i.e. the distance between the point of intersection of the beam of rays with the height h of incidence and the point of intersection of the paraxial beam of rays. This intersection length difference or the aperture aberration is a function of h and increases with increasing distance from the optical axis of the lens. In principle, there can be a deviation of the points of intersection parallel to the optical axis of the lens and also transversely with respect to the optical axis of the lens. In so far as an intersection length difference is mentioned in the context of this application and hereinafter, however, this only means the intersection length difference parallel to the optical axis of the lens, i.e. the longitudinal deviation. The latter is a measure of the aperture aberration.
The aperture aberration increases with increasing distance h from the optical axis of the lens. To an approximation, it increases in quadratic dependence on h. Consequently, the aperture aberration becomes relevant only at a certain distance from the optical axis. However, this is the case precisely for high-aperture lenses for cameras. High-aperture lenses in the context of the present application are understood to be those lenses in which the f-number can be set or is set to be ≦3. The f-number is the quotient of the set focal length of the lens and the diameter of the entrance pupil. In this case, the entrance pupil is dependent on the setting of a stop provided in the lens, for example of an aperture stop formed as an iris stop. If the f-number is small, this means that the stop provided in the lens is wide open. In this case, the distance h of a marginal ray still passing through the stop, i.e. of the furthest outwardly situated light ray of a light beam proceeding from a specific object point, is large. The aperture aberration is then also large for this distance. Consequently, in the case of a wide open stop, a larger proportion of light rays proceeding from an object point and impinging on the image plane in the camera have an aperture aberration; the latter is furthermore very large for the marginal rays. That can be counteracted by setting the stop aperture to be small. However, this is not always desired or possible, for example on account of the external light conditions.
In cameras, filters of different thicknesses are often provided upstream of the image plane. In particular, in digital cameras, a low-pass filter is provided upstream of a recording image sensor, for example a CCD sensor, in order to suppress the so-called Moiré effect. In the Moiré effect, undesirable coarse grids occur as a result of the superimposition of two fine grids in the context of an interference pattern. This Moiré effect is known in principle to the person skilled in the art. As stated, it can be suppressed by the use of a low-pass filter. Furthermore, other filters can also be provided in a camera, for example for filtering infrared radiation. The filters have different thicknesses depending on the type of camera. Furthermore, it may be provided that the number and type of the filters are changeable in a camera of a specific type, thus resulting overall in a different thickness of the filters used. One example of a similar camera is disclosed in document DE 100 28 233 A1.
One problem is that such a filter element of specific thickness d also causes a change in the intersection length. Since the filters in the camera are usually situated directly upstream of the image recording sensor, they lie in the convergent beam path and thus cause a lengthening of the intersection length. Since the thickness d and the filters of a camera or the equivalent glass thickness composed of the different refractive indices and thicknesses of the individual filters differs from camera type to camera type, a specific lens can be used with a specific camera type or only with camera types whose filter bundle has the same equivalent glass thickness. A lens is always designed such that it compensates for the lengthening of the intersection length caused by the filters of a specific camera type to be used together with the lens. On the image sensor, the aperture aberration is then completely compensated for. In the case of use together with a different camera type, this compensation is then no longer correct and a significant aperture aberration occurs particularly in the case of a low f-number or wide stop aperture.
In document US 2009/0052064 A1, a plane-parallel optical element of specific thickness is taken into account in the design of the lens element system. The thickness of said plane-parallel optical element is intended to be thicker than the actual thickness of a filter used. Depending on an equivalent glass thickness of an actual filter, a correspondingly thinner plane-parallel optical element than was taken into account in the design is then intended to be provided in the lens. This means, however, that the corresponding plane-parallel optical element provided in the lens is provided once for a specific camera type. If the lens is intended to be used together with a different camera type, it will be necessary to replace the plane-parallel optical element in the lens. However, not only is this time-consuming and complicated, but undesirable aberrations can also be caused, for instance if a new plane-parallel optical element is mounted incorrectly or is contaminated, or a plane-parallel optical element having an incorrect thickness is simply mounted inadvertently.
Therefore, it is an object of the present invention to specify a lens for a camera, more particularly a digital camera, which, even for an inexperienced user, is easily adaptable to a variable filter thickness and can thus be used with different camera types without any problems.